A "plumb line" is a string with a weight at the end that is used to find or measure vertical or straight alignment. It helps show if something is perfectly straight up and down. Full definition
Look around water fixtures such as under sinks or along main plumbing lines, since leaks in these areas can spread. (greenresidential.com)
They form connections, known as gap junctions, that send tendrils much like plumbing lines into the egg. (discovermagazine.com)
Now drop an imaginary plumb line from the top of the rope, and the big triangle can be divided into two smaller and equal right triangles, each with a hypotenuse of 180.5 feet and sides of 180 feet and h feet. (scientificamerican.com)